In conventional manufacture of semiconductor devices, it is well known to perform, for example, transportation of a wafer from one processing room to the other processing room through a dedicated path or directly so as to perform a predetermined sequential processing to the wafer. In this case, there are many opportunity to perform a variety of handling such as grasping, or transportation of the wafer. In particular, instruments and the like contacting the-wafer during handling are generally made of fluorocarbon resin, quartz or the like so as to avoid the wafer from being contaminated by metal. Therefore, when the wafer contacts the instruments, the wafer tends to be positively charged and to be at a high potential because of the electrification rank relationship with respect to the instruments.
The wafer is charged positively or negatively according to electrification rank of the contacting material with respect to the wafer as follows: the wafer is charged within a range from +500 to +3300 [V] when grasping the wafer by a pincette made of fluorocarbon resin, and is charged within a range from +600 to +2000 [V] when mounting the wafer on a stand made of polypropylene. Further, the wafer is charged within a range from +1000 to +1500 [V] when mounting the wafer on a quartz plate by using the pincette made of fluorocarbon resin, and is charged within a range from +500 to +3300 [V] when washing the wafer by extrapure water. Moreover, the wafer is charged within a range -200 to -1000 [V] when spraying gaseous nitrogen on the wafer.
However, if the wafer or the carrier is charged, suspended particles may be adhered to the wafer or the carrier by electrostatic force, or the semiconductor devices, during manufacturing, may be destroyed due to electrostatic discharge.
FIG. 9 illustrates the amount of suspended particles adhered by the electrostatic force to the wafer per surface potential of the wafer. There is shown the result of experiment in which a 5 inch wafer is employed as the wafer, and the wafer is vertically mounted through an insulating stand on a conductive grating. In FIG. 9, the transverse axis shows a potential of the wafer, and the ordinate shows the number of adhering particles. Here, the number of adhering particles means the number of particles adhering to effective area of the wafer (which is 69.4 [cm.sup.2 ] for a 5 inch wafer). It will be appreciated that the number of the adhering particles is converted to the number of particles adhering to the effective area, provided that the wafer is left for five hours in an atmosphere containing ten particles having a particle diameter of 0.5 [.mu.m] or more per unit volume (cubic feet).
Referring to FIG. 9, there is little accumulation of particles due to gravity settling in case the wafer is vertically mounted. Accordingly, no particle adheres to the wafer when the wafer potential is low, i.e., at a range of 0 to 50 [V]. However, the number of the adhering particles is rapidly increased due to the effect of electrostatic force as the wafer potential rises in a range of 300 to 1800 [V].
FIG. 10 illustrates an adhering range of the suspended particles by the electrostatic force with a particle diameter used as a parameter provided that high voltage (1000 [V]) is applied to the wafer W. In this case, a particle concentration is set to 1 [g/cm.sup.3 ]. Further, in FIG. 10, the rectangular closing line shows potential 0, the dotted lines in the closing line show equipotential lines, and the solid lines show lines of electric force.
Referring to FIG. 10, there are little particles adhering to the wafer when particles have a particle diameter of 2 [.mu.m]. As the particle diameter is decreased to 0.5 [.mu.m] or 0.1 [.mu.m], the adhering range is rapidly extended. That is, it will be appreciated that the smaller the particle diameter of the suspended particles, the easier the particles are adhered to the wafer due to the electrostatic force.
There are two well-known methods to avoid the wafer or wafer carrier from being charged as follows: one is a method employing a so-called ionizer, that is, a method for generating corona discharge in an atmosphere where the wafer or the wafer carrier is placed and neutralizing the resultant ions with the charge. The other is a method for handling the wafer by using a grounded metallic body or a resin material containing grounded conductive materials (carbon, metal and the like) so as to discharge the charge.
However, the corona discharge in the atmosphere is conventionally utilized in the first method. Consequently, positive ions in generated ions are mainly ions of water (H.sub.2 O)nH+ so that the water ions (H.sub.2 O)nH+ develop a growth of natural oxide film on the wafer surface. Further, negative ions in the generated ions are mainly CO.sub.3 -, NO.sub.x - and SO.sub.x -so that these negative ions also cause formation of a natural oxide film because of the strong oxidizing force thereof as in the case of the positive ions.
On the other hand, the wafer conventionally directly contacts the metal or the conductive materials in the second method. Hence, particles of the metals or the conductive materials are adhered to the wafer, resulting in dark current or leakage current.